This invention relates to blends of linear aromatic polyesters and linear aromatic polysulfonates. More particularly it is directed to polyblends of polyesters derived from a bisphenol and a dicarboxylic acid with polysulfonate polymers derived from a bisphenol and a disulfonic acid. It is especially concerned with solid solution polyblends of the foregoing polymeric components.
Linear aromatic polyesters prepared from dicarboxylic acids, especially from aromatic dicarboxylic acids and bisphenols are well known for their suitability for molding, extrusion, casting, and film-forming applications. For example, U.S. Pat. No. 3,216,970 to Conix, disclose linear aromatic polyesters prepared from isophthalic acid, terephthalic acid, and a bisphenolic compound. Such high molecular weight compositions are known to be useful in the preparation of various films and fibers. Further, these compositions, when molded into useful articles using conventional techniques, provide properties superior to articles molded from other linear polyester compositions. For instance, aromatic polyesters are known to have a variety of useful properties, such as good tensile, impact, and bending strengths, high thermal deformation and thermal decomposition temperatures, resistance to UV irradiation and good electrical properties.
Aromatic polyesters which are particularly well suited for film-forming and molding applications may also be prepared by reacting an organic diacid halide with a difunctional aliphatic reactive modifier, such as a glycol, and subsequently reacting this product with a bisphenol compound. The resulting polyesters have reduced melt viscosities and melting points which permits molding at temperatures within the operable limits of conventional molding appartus (i.e. less than about 300.degree. C.). This type of glycol-modified polyester is more fully disclosed in U.S. Pat. No. 3,471,441, to Hindersinn et al.
In order to form a successful molding resin on a commercial scale, a polymer should be capable of being molded conveniently without significant degradation in physical properties. In this respect, although the aforementioned aromatic polyesters generally display excellent physical and chemical properties, a persistent and troublesome problem has been their sensitivity to hydrolytic degradation at elevated temperatures. This sensitivity to the combined effects of heat and moisture is also exhibited in commercially available polycarbonate resins as evidenced by the desirability of reducing the water content of the resin to less than about 0.05% prior to molding. Unfortunately, however, the aromatic polyester resins often display a more pronounced tendency to rapidly degrade and embrittle than do polycarbonate resins. This is demonstrated by the loss in a polymer property, e.g. tensile strength, which can occur when an aromatic polyester resin is molded and subsequently immersed in boiling water. This tendency may be explained, in part, by the hydrolysis of the ester linkages under these conditions. In any event, it is to be appreciated that sensitivity to moisture represents a significant problem in aromatic polyester resins that would significantly limit their commercial utility in applications such as in autoclaves or at elevated temperatures in humid atmospheres.
In addition to the aforementioned problem of hydrolytic stability, the linear aromatic polyesters have relatively poor bending flexibility, i.e. films of the polyester readily crack on repeated bending.
It is known that the linear aromatic polysulfonates have relatively poor hydrolytic stability compared to polyesters of carboxylic acids (Encyclopedia of Polymer Science and Technology, Interscience Vol. 11, 1969, pg. 77 and P. W. Morgan "Condensation Polymers" Interscience Publishers, 1965, p. 384). Accordingly, addition thereof to a linear aromatic polyester would not be considered as a likely method of improving the hydrolytic stability of the latter polyester. Therefore, it was surprising and unexpected to find in accordance with this invention that polyblends of the aromatic polyesters and aromatic polysulfonates have enhanced hydrolytic stability. It is noted that U.S. Pat. No. 3,262,914 (to E. P. Goldberg et al) teaches that a copolyester containing the monomer residue of a bisphenol together with an aromatic dicarboxylic acid residue and an aromatic disulfonic acid residue has good resistance to hydrolysis by aqueous ammonia, caustic, and mineral acid. In the patent, the method of determination of hydrolytic stability is by weight loss after contact with the aqueous corrosive reagent. As pointed out below, this technique is less appropriate for measurement of hydrolytic stability than measurement of loss of intrinsic viscosity by the polymer on immersion in water of neutral pH, which is the method employed to measure hydrolytic stability in the present invention.
Accordingly, it is a principal object of this invention to prepare aromatic polyester compositions having enhanced hydrolytic stability and flexibility as well as other excellent chemical and physical properties.